During rework of a circuit board, the process of removing components from the board, two conflicting goals must be accomplished. First, the proper heat must be maintained to allow the solder securing the components to the circuit board to reach and remain at the reflow temperature. Secondly, it is necessary to prevent the applied heat from dispersing through the circuit board to adjacent components and causing potentially damaging stress to the circuit board or card and components attached thereto.
It is common to mount complex electronic components on printed boards by inserting pins extending from components into plated through holes in the circuit board and soldering them in place. The plated through holes normally provide connections between the pins and conductive material situated at various levels of the circuit board or card.
As the number of components attached to the circuit board increases, both the number of pins and the number of times the board must be heated to solder the pins in place increase. Therefore, during rework, a great amount of heat is potentially applied to a circuit board or card.
As the circuit board increases in size and thickness, the total amount of copper and the number of planes common to a single via all contribute to insufficient soldering and the inability to rework an assembly. Further, the soldering process is adversely effected when tied planes in a circuit board or card, that is, planes in the circuit board or card that are electrically connected to the through hole, allow heat being applied to the plated through hole to escape into the internal planes of the circuit board. The escape of heat from the through hole reduces the through hole temperature to below the solder melting point resulting in insufficient hole fill.
The draining of heat from the through hole is especially prevalent when two or more planes are common to a single through hole. This allows heat to escape into the internal planes of the circuit board or card from the plated through hole, thereby inhibiting the top surface of the carrier from reaching the solder reflow temperature. Thicker circuit boards that do not have common power planes also experience rework problems. It is also essential that when heat is applied to the circuit board during rework that the temperature not exceed the melting point of the material used to form the circuit board. However, enough heat must be applied to the solder to cause the solder to melt throughout the length of the plated through hole.